The present invention relates to a velocimeter which irradiates laser light onto a moving measuring object and receives scattered light from the measuring object and measures an optical frequency shift quantity corresponding to the moving speed of the measuring object so as to detect the speed of the measuring object.
As a general rule, when a light source and an observer move relative to each other, light is subjected to frequency changes by the Doppler effect. The laser Doppler velocimeter (hereinafter, referred to as LDV) uses this effect to measure the moving speed of a measuring object by irradiating laser light onto the object and measuring a Doppler frequency shift of the scattered light derived from the measuring object. This laser Doppler velocimeter, which was disclosed by Yeh and Cummins in 1964 (Appl. Phys. Lett. 4–10 (1964) 176), is widely known and is in practical use today.
FIG. 15 shows an optical system diagram of a conventionally typical LDV.
In FIG. 15, 101 denotes a laser diode (hereinafter, referred to as LD) as a semiconductor laser, 102 denotes a photodiode as a photodetector (hereinafter, referred to as PD), 103 denotes a diffraction grating, 104 denotes a collimator lens (hereinafter, referred to as CL), 105 denotes a mirror, 106 denotes a condenser lens, 107 denotes a first luminous flux of +first order diffracted light by the diffraction grating 103, 108 denotes a second luminous flux of −first order diffracted light by the diffraction grating 103, and 113 denotes a measuring object.
In the optical system as constituted above, laser light emitted from the LD 101 is converted by the CL 104 into a parallel luminous flux, and then is split into ±first order diffracted lights at a diffraction angle of θ by the diffraction grating 103 to become the first luminous flux 107 and the second luminous flux 108. The first luminous flux 107 and the second luminous flux 108 are respectively reflected by the mirror 105 and are then made incident on the surface of the measuring object 113 at an incident angle of θ to be overlapped each other again. The first luminous flux 107 and the second luminous flux 108 scattered by the measuring object 113, which are Doppler frequency-shifted, are slightly different from the LD 101 in oscillating frequency. As a result, the interferential waves of the first luminous flux 107 and the second luminous flux 108 scattered by the measuring object 113 generate beat. This beat is termed beat signal. The moving speed of the measuring object 113 is obtained by heterodyne-detecting the beat frequency of the beat signal using the PD 102. Hereinafter, this conventionally typical LDV will be described in further detail.
Here, when the direction in which the measuring object 113 moves to the right, as shown in FIG. 15 is set as the normal direction, the frequency shift of the first luminous flux 107 is Doppler frequency-shifted by −fd and the second luminous flux 108 is Doppler frequency-shifted by +fd, so that the apparent frequency of the first luminous flux 107 becomes (f0−fd) and the apparent frequency of the second luminous flux 108 becomes (f0+fd). Note that f0 represents the oscillating frequency of the LD 101. In this case, since an electric field of the light emitted from the LD 101 is represented as E0·cos (2πf0t), the first luminous flux 107 is indicated by the following Equation 1 and the second luminous flux 108 by the following Equation 2:IA=EA·cos {2π(f0−fd)t+φA}  (1)IB=EB·cos {2π(f0+fd)t+φB}  (2)Note that f0 denotes the frequency of outgoing beam from the LD 101, E0 denotes the amplitude of the outgoing beam from the LD 101, EA denotes the amplitude of the first luminous flux 107, EB denotes the amplitude of the second luminous flux 108, φA the phase of the first luminous flux 107 and φB denotes the phase of the second luminous flux 108.
Since the frequency of light is generally 100 THz (1014 Hz), it is impossible to measure the frequency information of Equation 1 and Equation 2 directly. Therefore, since heterodyne detection is generally employed for direct measurement as mentioned above, and f0>>fd is established, the interferential waves of Equation 1 and Equation 2 are indicated by the following equation:
                              〈                                                                                    I                  A                                +                                  I                  B                                                                    2                    〉                =                                                            E                A                2                            +                              E                B                2                                      2                    +                                                    E                A                            ·                              E                B                            ·              cos                        ⁢                          {                                                2                  ⁢                                                                          ⁢                                      π                    ⁡                                          (                                              2                        ⁢                                                  f                          d                                                                    )                                                        ⁢                  t                                -                                  (                                                            ϕ                      A                                        -                                          ϕ                      B                                                        )                                            }                                                          (        3        )            Note that < > in the left side of Equation 3 represents time average. Consequently, the PD 102 allows the frequencies of these interferential waves to be measured.
FIG. 16 shows a case in which the measuring object 113 moves at a speed of V, two luminous fluxes are made incident on the object 113 at arbitrary angles of α and β respectively, and the observation point receives scattered light at an arbitrary angle of γ.
Frequency shift quantity due to the Doppler effect, which is obtained using the Lorentz transformation based on relativism in a precise sense, is approximately obtained as follows when the moving speed V is sufficiently smaller than speed of light c. Relative velocities VA1 and VB1 of light from a light source A and a light source B and a moving object is indicated by the following equations:VA1=c−V sin αVB1=c+V sin β  (4)Also, apparent frequencies fA1 and fB1 of the respective lights seen from the measuring object 113 are indicated by the following equations:
                                          f            A1                    =                                                    V                A1                            λ                        =                                          1                λ                            ·                              (                                  c                  -                                      V                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    α                                                  )                                                    ⁢                                  ⁢                              f            B1                    =                                                    V                B1                            λ                        =                                          1                λ                            ·                              (                                  c                  +                                      V                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    β                                                  )                                                                        (        5        )            Relative velocities VA2 and VB2 of the respective scattered (reflected) lights and the measuring object 113 are indicated by the following equations:VA2=c−V sin γVB2=c−V sin γ  (6)Consequently, frequencies fA2 and fB2 of lights seen from the observation point are indicated by the following equations:
                                          f            A2                    =                                                    c                                  V                  A2                                            ·                              f                A1                                      =                                          c                λ                            ·                                                1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    α                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                      ⁢                                  ⁢                              f            B2                    =                                                    c                                  V                  B2                                            ·                              f                B1                                      =                                          c                λ                            ·                                                1                  +                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    β                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                                          (        7        )            The difference between the frequency in Equation 7 and the frequency of incident light (f0) becomes a Doppler frequency shift quantity fd. Here, the beat frequency of the two luminous fluxes measured at the observation point 2fd is indicated by the following equation using c>>V:
                                                                        2                ⁢                                  f                  d                                            =                                                                                    f                    B2                                    -                                      f                    A1                                                                                                                                          =                                                V                  λ                                ·                                  (                                                            sin                      ⁢                                                                                          ⁢                      α                                        +                                          sin                      ⁢                                                                                          ⁢                      β                                                        )                                                                                        (        8        )            It can be seen that 2fd is independent of a position of the observation point (angle: γ). In FIG. 15, in which α=β=θ is valid, the following equation is established based on Equation 8 according to the typical optical system of the LDV of FIG. 15:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·            sin                    ⁢                                          ⁢          θ                                    (        9        )            Consequently, the moving speed V of the measuring object 113 is obtained by measuring the frequency 2fd indicated in Equation 3 and calculating using Equation 9.
It is also possible to present Equation 9 geometrically as follows: FIG. 17 is an enlarged view of an area in which the two luminous fluxes in FIG. 15 (the first luminous flux 107 and the second luminous flux 108) overlap each other again. The two luminous fluxes intersect at θ incident angles respectively, and the broken lines in FIG. 17 show parts of the equal wave surfaces of the respective luminous fluxes. An interval between the broken lines shows the wavelength of light λ. The vertical heavy lines show the bright parts of interference fringes, and when the interval between the vertical heavy lines is set as Δ, this Δ is indicated by the following Equation 10:
                    Δ        =                  λ                      2            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              (        10        )            
As shown in FIG. 17, when an object (shown as •) passes perpendicularly to the interference fringes at a velocity of V, the frequency f is indicated by the following equation:
                    f        =                              V            Δ                    =                                                                                          2                    ⁢                    V                                    λ                                ·                sin                            ⁢                                                          ⁢              θ                        =                          2              ⁢                                                          ⁢                              f                d                                                                        (        11        )            This equation is made equal to Equation 9.
The moving speed V of a typical LDV is thus obtained; however, it is impossible to detect the moving direction of a measuring object. In contrast, detecting a moving direction is made possible by rotating the diffraction grating 103 in FIG. 15 at a velocity of Vg (see JP H03-235060A). As a result, when light is reflected by the diffraction grating 103, since the respective luminous fluxes are Doppler frequency-shifted in proportion to Vg, the beat frequency 2fd to be measured in the PD 102 is obtained by the following equation:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·                          (                              V                +                                  V                  g                                            )                        ·            sin                    ⁢                                          ⁢          θ                                    (        12        )            Consequently, a moving direction is obtained since the magnitude correlation of 2fd is determined according to the positive and negative signs of the moving speed V relative to a given velocity of Vg. According to the abovementioned optical system, however, a rolling mechanism of the diffraction grating 103 is required with result that the device becomes larger in size and higher in cost. In addition, the diffraction grating 103, the rotational speed of which needs to be precisely maintained, is difficult to employ for precise measurement due to problems such as error caused by eccentricity and so on, and vibration and so on caused by rotation.
A velocimeter which solves the above problems is disclosed in JP H04-204104A. In JP H04-204104A, the moving direction of a measuring object is detected by using a frequency shifter to change the frequency of an incident luminous flux.
FIG. 18 shows an optical schematic diagram of the velocimeter disclosed in JP H04-204104A.
According to the velocimeter, light emitted from a laser source 1 become a parallel luminous flux by a CL 104, and then are split into two luminous fluxes by a beam splitter (hereinafter, referred to as BS) 109. The luminous fluxes are reflected by a mirror 105 and are then frequency-shifted by f1 and f2 by an acousto-optic device (hereinafter, referred to as AOM) 110. The light is again collected on the surface of a measuring object 113 by a diffraction grating 103 so as for the beat frequency of scattered light from the measuring object 113 to be detected using a PD 102. The frequency 2fd to be detected here is indicated by the following equation:
                              2          ⁢                      f            d                          =                              (                                                                          f                  1                                -                                  f                  2                                                                    )                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        13        )            Consequently, the moving direction of the measuring object 113 is detected by the magnitude correlation of 2fd relative to a given frequency shift quantity |f1−f2| since the sign of V changes according to the moving direction of the measuring object 113.
Also in JP H08-15435A, frequency is changed using an electro-optical device (hereinafter, referred to as EOM) 111 shown in FIG. 19 based on the same principle as the principle employed in JP H04-204104A. More specifically, light emitted from a LD 101, which is a laser source, becomes a parallel luminous flux by a CL 104, and is then split into two luminous fluxes, a first luminous flux 107 and a second luminous flux 108, by a diffraction grating 103. The first luminous flux 107 and the second luminous flux 108 are together made incident on the EOM 111. Here, bias is applied to the second luminous flux 108 to shift frequency by fR. The first luminous flux 107 and the second luminous flux 108 are reflected by a mirror 105, and then are collected on the surface of the measuring object 113. The beat frequency of scattered light from the surface of the measuring object 113 is detected using a PD 102. The frequency 2fd to be detected here is indicated by the following equation:
                              2          ⁢                      f            d                          =                              f            R                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        14        )            Consequently, similarly to Equation 13, the moving direction of the measuring object 113 is detected by the magnitude correlation of 2fd relative to a given frequency shift quantity fR since the sign of V changes according to the moving direction of the object.
However, an optical system where the moving direction of the measuring object 113 is detected using frequency shifters such as the AOM 110 and the EOM 111, is disadvantageous in that the device is made larger in size since the optical system becomes more complex and facilities for driving the frequency shifters such as a power source are required, for example, voltage necessary to frequency-modulate by the AOM 110 is about tens of volts and voltage necessary to frequency-modulate by the EOM 111 is about 100 volts with the result that a large-sized power source is required.
Requests for device miniaturization and lower power consumption of various sensors including the LDV have increased, and this tendency is particularly strong for consumer products. Since the LDV detects scattered light, signal light from a measuring object is generally weak and may be different according to a type of the object. One solution is to employ a photomultiplier tube as a photodetector having high photosensitivity, but a photomultiplier tube employed for the LDV causes the device itself to be jumboized. That is, the LDV provided with a photomultiplier tube is not suitable for application to small-sized consumer products. Instead, a photodiode, which is inferior as a photodetector in photosensitivity, is generally employed, so as not to obstruct device miniaturization. Accordingly, as much signal light as possible is preferably made incident on the photodetector. However, there is a limitation on the light reception system simply becoming closer since the distance between the light scattering surface on the measuring object 113 and a condenser lens 106 is usually limited due to factors such as arrangement of optical components. As another measure for as much signal light as possible to be made incident on the photodetector, it is also possible to increase incident light quantity by employing gas lasers and so on of He—Ne and Ar+ as a high-power laser source, but a semiconductor laser is preferred from the viewpoint of device miniaturization and lower power consumption.